The Brexit Party was the clear winner, but the Lib Dems and the Green Party also made significant gains.
The Conservatives have come in fifth place, with less than 10% of the vote.
Voting took place against a backdrop of paralysis at Westminster over Brexit, and the two biggest parties – Conservatives and Labour – had been damaged by their various contortions on the issue.
They were beaten by rivals – on both the Leave and Remain sides – who offered clarity while they have tried to find nuanced ways through, she added.
Polling expert Sir John Curtice said the results demonstrated how polarised the country was on Brexit – and how evenly support is split between Remain and Leave.
Tory leadership: Johnson says party on ‘final warning’
Boris Johnson, the front-runner in the Tory leadership race, has said the party risks being “dismissed” if it does not deliver Brexit. He said voters in the EU election issued a “crushing rebuke” to the Conservatives.
Fellow candidate, Foreign Secretary Jeremy Hunt, said the party faces an “existential risk” over Brexit.
Eight candidates have declared they are standing for leader, after Theresa May said she would resign.
Mr Johnson said voters had issued the party with a “final warning” as the Tories came fourth in Hillingdon, where he is an MP, and the Brexit Party emerged with the largest number of MEPs overall.
Senior Labour figures have called for an urgent change in the party’s Brexit policy after “disastrous” EU election results.
As Labour fell to third place, deputy leader Tom Watson said: “We need a change of direction urgently.”
Shadow foreign secretary Emily Thornberry said Labour should now campaign to remain in the EU.
But Labour leader Jeremy Corbyn said the party would “bring our divided country together”.
With some results still to declare, Labour was on course for less than 15% of the vote – worse than the party’s previous low in 2009.